The present invention relates a method for carrying out a plasma deposition process including the steps of:                i) providing a hollow substrate tube;        ii) supplying a supply flow of dopant-containing glass-forming gases to the substrate tube of step i), wherein the supply flow comprises a main gas flow and one or more secondary gas flows, preferably said main gas flow mainly comprising the glass-forming gases and said one or more secondary gas flows mainly comprising precursors for dopant(s);        iii) inducing a plasma by means of electromagnetic radiation in at least a part of the substrate tube of step ii) to create a reaction zone in which deposition of one or more glass layers onto the interior surface of the substrate tube takes place;        iv) moving the reaction zone back and forth in longitudinal direction over the substrate tube between a reversal point located near the supply side and a reversal point located near the discharge side of said substrate tube; wherein each forth and each back movement is called a stroke;        wherein the flow of at least one secondary gas flow is interrupted one or multiple times during step iii); each of said interruptions having a start point and an end point as a function of the axial position of the plasma along the length of the substrate tube.        
Plasma-enhanced chemical vapor deposition (PECVD or PCVD) is a process used to deposit thin films from a gas state (vapor) to a solid state on a substrate. Chemical reactions are involved in the process, which occur after creation of a plasma of the reacting gases.
Generally, in the field of optical fibers, multiple thin films of glass are deposited on the inside surface of a substrate tube. Glass-forming gases (viz. doped or undoped reactive gases) are introduced into the interior of the substrate tube from one end (supply side of the substrate tube). Doped or undoped glass layers are deposited onto the interior surface of the substrate tube. The gases are discharged or removed from the other end of the substrate tube, optionally by the use of a vacuum pump (discharge side of the substrate tube). The vacuum pump has the effect of generating a reduced pressure in the interior of the substrate tube, which reduced pressure generally comprises a pressure value ranging between 5 and 50 mbar.
Generally, electromagnetic radiation from generator is directed towards an applicator via a waveguide, which applicator surrounds a substrate tube. The applicator couples the electromagnetic energy into the plasma. The applicator (and hence the plasma formed by that) is moved reciprocally in the longitudinal direction of the substrate tube, as a result of which a thin glass layer is deposited onto the interior of the substrate tube with every stroke or pass.
The applicator and the substrate tube are generally surrounded by a furnace so as to maintain the substrate tube at a temperature of 900-1300° C. during the deposition process.
Thus the applicator is moved in translation over the length of the substrate tube within the boundaries of the furnace. With this translational movement of the resonator the plasma also moves in the same direction. As the resonator reached the inner wall of the furnace near one end of the substrate tube, the movement of the resonator is reversed so that it moves to the other end of the substrate tube towards the other inner wall of the furnace. The resonator and thus the plasma travels a back and forth movement along the length of the substrate tube. Each movement of the applicator from one reversal point to another reversal point is called a “pass” or “stroke”. With each pass a thin layer of glass is deposited on the inside of the substrate tube.
Normally, a plasma is generated only in a part of the substrate tube, viz. the part that is surrounded by the microwave applicator. The dimensions of the applicator are smaller than the dimensions of the furnace and of the substrate tube. Only at the position of the plasma, the reactive gasses are converted into solid glass and deposited on the inside surface of the substrate tube.
When the number of passes increases the cumulative thickness of these thin films, i.e. of the deposited material, increases thus leading to a decrease in the remaining internal diameter of the substrate tube. In other words, the hollow space inside the substrate tube keeps getting smaller with each pass.
One way of manufacturing an optical preform by means of a PCVD process is known from U.S. Pat. No. 4,314,833. According to the process that is known from that document, one or more doped or undoped glass layers are deposited onto the interior of a substrate tube, using a low-pressure plasma in the glass substrate tube. After the glass layers have been deposited onto the interior of the glass substrate tube, the glass substrate tube is subsequently contracted by heating into a solid rod (“collapsing”). In a special embodiment, the solid rod may furthermore be externally provided with an additional amount of glass, for example by means of an external vapor deposition process or by using one or more preformed glass tubes, thus obtaining a composite preform. From the preform thus produced, one end of which is heated, optical fibers are obtained by drawing.
According to International application WO 99/35304 in the name of the present applicant, microwaves from a microwave generator are directed towards an applicator via a waveguide, which applicator surrounds a glass substrate tube. The applicator couples the microwave energy into the plasma.
U.S. Pat. No. 4,741,747 relates to methods for reducing optical and geometrical end taper in the PCVD process. The regions of non-constant deposition geometry at the ends of the preform (taper) are reduced by moving the plasma in the area of at least one reversal point nonlinearly with time and/or by changing the longitudinal extent of the plasma as a function of time.
EP 2 573 056 relates to a method for manufacturing a primary preform having a reduced taper.
From European patent application EP 2199263 (also published as US 2010/0154479) by the present applicant is known a PCVD process which can be used to minimize axial refractive index variations along a substrate tube by controlling the gas composition (primarily dopant composition) in the substrate tube as a function of the resonator (plasma zone) position. This system is complicated to build and maintain.
From European patent application EP 2 377 825 of the present applicant is known a process for manufacturing a primary preform wherein pulses of a fluorine containing gas are supplied when the reaction zone is at the reversal point.
From European patent application EP 2 594 659 of the present applicant is known an apparatus for carrying out a PCVD deposition process, wherein one or more doped or undoped glass layers are coated onto the interior of a glass substrate tube, which apparatus comprises an applicator having an inner wall and an outer wall and a microwave guide which opens into the applicator, which applicator extends around a cylindrical axis and which is provided with a passage adjacent to the inner wall, through which the microwaves supplied via the microwave guide can exit, over which cylindrical axis the substrate tube can be positioned, whilst the applicator is fully surrounded by a furnace that extends over said cylindrical axis.
From European patent application EP 1,923,360 (also published as US 2009/0022906) of the present applicant a PCVD process is known which provides uniform thickness and refractive index deposition in the axial direction of the substrate tube. In this method the furnace is moved reciprocally, e.g. 30 mm, 60 mm, or 15 mm, along the axial direction of the substrate tube. The movement of the furnace is used to reduce the effect of what is believed to be non-uniform distribution of microwave power along the axial direction of the substrate tube, caused by microwave applicator position-dependent reflections of some of the microwave power, e.g. from the inner wall of the surrounding furnace. Such axial microwave power non-uniformity can cause axial deposition thickness and refractive index non-uniformity, which adversely affects fiber quality parameters such as attenuation, mode field width uniformity, and bandwidth uniformity.